The load capacity of mechanical elements that roll on each other, such as those employed in tractive friction drives, ball and roller bearings, roll-followers for cams, etc., is governed by the maximum allowable contact pressure. In order to maximize this allowable surface pressure, these rolling contact elements have usually been made of the highest strength through-hardened or case-hardened steel. This is because allowable contact pressure is approximately a linear function of the "modulus of resilience", and no other material has heretofore been able to match the modulus of resilience of hardened steel.
The hardness of steel, however, especially when it is quenched and tempered, varies greatly with size. When a large steel item is quenched, the interior retains so much heat after the exterior has solidified that much of the hardness of the case is drawn before an equilibrium temperature is reached. As a result, while a small steel plate can be hardened to give a residual compressive stress in the case, parallel to the surface, of about 100,000 psi (703 MPa), the corresponding stress in a large piece of quenched steel is more of the order of 50,000 psi (352 MPa). The importance of this difference in residual stress with respect to allowable contact loads will be discussed below.
Accordingly, the object of the invention is to increase the allowable surface load on rolling contact elements. The means disclosed are particularly applicable to larger steel elements and elements made of materials that have limited Hertzian strength or hardenability.
The means to achieve these and other objects and advantages of the invention will be evident from the drawings as explained in the specification that follows.